In U.S. Pat. No. 4,378,159, owned by the assignee of this application, L. K. Galbraith describes a scanning laser contaminant and defect detector which uses a light collector for increasing the sensitivity of the detector. In other words, the effective aperture of the detector is modified by use of a light collector to gather diffusely reflected light.
In the prior device, the collector is a quadrant of a diffusely reflective spherical shell cradled between V-shaped, specularly reflective side walls. The shell has beam entrance and exit ports, as well as a detector port where a light detector resided. The collector is placed in proximity to a test surface to be inspected. Light scattered from the test surface is directed to the interior of the reflective shell surface, then to the reflective side walls and ultimately to the detector.
One of the problems of prior laser scanners was unwanted scattered light; i.e., scattered from other than the target surface, reaching the detector. For example, dirt particles on the scanning mirror can scatter light. This light behaves as if it originates at a new source having properties different from the main beam. The unwanted light has a high probability of collection at the detector once it enters the light collector. The unwanted light is considered to be optical "noise," lowering the overall signal-to-noise ratio of the instrument.
An object of the invention was to achieve increased sensitivity in a scanning contaminant and defect detector by limiting the effects of light scattered from other than the target surface.